Unlike the early sauropsids, Hylonomus and its kin, Archaeothyris was relatively large, measuring 50 centimetres (20 in) head to tail. It was also more advanced than the early sauropsids (~reptiles), with strong jaws that could open wider than those of the sauropsids. Its sharp teeth were all of the same shape, and it had a pair of enlarged canines, suggesting that it was a carnivore.[2]